2,3,5,6-Tetrachloropyridine (symmetrical tetrachloropyridine--"sym-tet", henceforth) is an important intermediate for the preparation of several commercial pesticides. A variety of methods of preparing it are known; these methods preponderantly involve chemical or electrolytic reduction of pentachloropyridine ("PCP" hereinafter). None of the known methods are "electrocatalytic", as the term is used here.
All electrochemical reactions are catalytic in a sense, since the primary step, the electron exchange, is "catalyzed" by the electrode. Some electrochemical reactions involve the formation of a reactive intermediate species in this step, such as nascent hydrogen--for example, which then reacts chemically with the substrate, as by reduction, for example. If the source material for the reactive species is not regenerated in the cell, the overall process is better described as indirect electroreduction (or oxidation) than as electrocatalytic. Here, the term electrocatalytic is reserved for processes which may be rationalized as involving the formation of a reactive species from a source material (Zn.sup.2+) which is regenerated in the course of the overall reaction.
Of the several known types of chemical reductions of PCP to sym-tet, the one most relevant to the present invention is the use of zinc metal in acidic media. British Pat. No. 1,499,650 is directed to such a process wherein a mixture of PCP, zinc particles, HCl and water is heated to a temperature of 110.degree.-160.degree. C. under at least autogenous pressure. The reaction is described in the patent as reduction of the PCP by nascent hydrogen generated by the reaction of the zinc with the acid. The highest reported yield of sym-tet, equivalent to 91.4% based on PCP charged, was obtained at a PCP conversion of 98.8%. A total of about 5% of di- and trichloropyridines and about 1% of an isomeric tetrachloropyridine were co-produced.
U.S. Pat. No. 4,259,495 is directed to another chemical method in which PCP is reduced with from 1.1 to 1.3 gram atoms of zinc per mole of the PCP in a mixture of a phosphonate or phosphate ester and a solution in water of about 2-3 moles/liter of an ammonium salt, such as ammonium carbonate, ammonium salts of phosphonomonesters, ammonium sulfate and ammonium chloride. The reaction is carried out at temperatures of 60.degree.-120.degree. C. under ambient pressures. It is stated that sym-tet can be obtained, 97% pure, in 92% of theoretical yield. PCP conversions of up to about 99% and trichloropyridine contents as low as about 1% are attained.
Electrolytic reductions of PCP to sym-tet are disclosed in two U.S. patents. U.S. Pat. No. 3,694,332 is directed to the use of mercury, lead, iron, tin or zinc cathodes for the reduction of PCP dissolved in an alcohol, a cyclic or acyclic ether, a glycol monoether, a lower amide or sulfolane. A neutral or acidic salt such as sodium or ammonium tosylate, acetate, chloride or fluoride or a free acid such as acetic acid, HCl or H.sub.2 SO.sub.4 is used as the electrolyte. The reaction product obtained at a stirred mercury cathode from a solution of PCP in dimethoxyethane and methanol, using 30% H.sub.2 SO.sub.4 as the electrolyte, in a period of 129 minutes at 25.degree. C. (current efficiency 88%), consisted of trichloropyridines (2.7%), sym-tet (97.3%) and a trace of PCP.
U.S. Pat. No. 4,242,183 discloses the use of a particularly active type of silver cathode to reduce PCP to sym-tet (or to 2,3,5-trichloropyridine). Yields of up to about 90% of the sym-tet can be attained, without substantial further reduction, if the reaction is stopped after about 96% of the PCP has been converted. The reduction is carried out in a basic, aqueous catholyte comprising an organic solvent such as THF, dioxane, lower alcohols, glycol ethers, sulfolane and lower amides. A porous separator is used to divide the cell into separate cathode and anode compartments, the latter being charged with an aqueous base. A cathode potential of about -1.1 volts (relative to a saturated calomel electrode) is preferred for sym-tet production; about -1.3 volts is preferred for 2,3,5-trichloropyridine production. Temperatures of about 20.degree.-35.degree. C. are preferred.
An electrocatalytic method of converting a vicinal dihalo-ethane or -propane to the corresponding ethylene or propylene and the elemental halogen(s) is disclosed in U.S. Pat. No. 4,162,948. A divided cell is used. The catholyte contains the dihalo compound, water, zinc chloride and a detergent. The anolyte (composition not disclosed) presumably consists of ZnCl.sub.2 and water. The dehalogenation is said to be operable at temperatures of from 0.degree.-100.degree. C.; examples at 40.degree. and 60.degree. are given. The electrolytic reaction disclosed is the decomposition of the ZnCl.sub.2 to metallic zinc (at the cathode) and chlorine (at the anode). The zinc chloride (or at least a zinc dihalide) is regenerated (or formed) as a product of the chemical dehalogenation reaction between the adjacent halogen substituents on the dihaloalkane and the metallic zinc. This process, like the classical coupling reaction between 2 molecules of a monohalocompound and a metal, does not result in replacement of a halosubstituent by hydrogen. (The coupling type of reaction is known to occur when 4-halo-2,3,5,6-tetrafluoropyridines are electroreduced in aprotic media: Chambers, Clark, Sargent and Drakesmith, Tetrahedron Letters, 21, p. 1917 (1979)).
Essentially the same system can be employed, according to Pletcher, Razaq and Smilgin; Jour. Appl. Electrochem., 11, (1981) 601-603, to effect either the conversion of 1,1,2-trichloroethane to vinyl chloride or of nitrobenzene (as such or as the 2-fluoro derivative) to aniline (or 2-fluoroaniline). The use of a two-phase system (i.e., an emulsion) is said to have permitted attainment of current densities greater than 0.5 A/cm.sup.2. The presence of HCl in amounts up to 7M did not adversely effect current efficiency for zinc deposition (in a preliminary experiment involving no organic substrate). Again, neither reaction results in replacement of a halo-substituent by hydrogen or removal of a halo substitutent from an aromatic (carbocyclic or heterocyclic) ring.
All of the foregoing methods of reducing PCP to sym-tet leave something to be desired, in one respect or another. The chemical methods of the British '650 patent and the U.S. '495 patent result in production of a zinc salt waste stream, which must be disposed of in an environmentally acceptable manner. The electrolytic method of the U.S. '332 patent requires, for really good results, the use of a mercury or lead cathode. Use of these metals, especially mercury, in electrolytic processes, without contaminating the environment, has been found by industry to be difficult. The electrolytic method of the U.S. '183 patent requires the use of an expensive (silver) cathode material which must be cleaned with acid (at the expense of some loss of silver) and reactivated (by anodization) frequently.
The electrocatalytic reactions described above are said, by Pletcher et al, to be illustrative of what can be done by electrocatalysis. However, which specific reactions of other types are and are not amenable to this approach is not indicated. Those familiar with the great and often unpredictable sensitivity of electrochemical systems to small differences in materials and conditions will appreciate that the foregoing prior art falls short of suggesting that electrocatalysis is a feasible alternative to the known electrolytic methods for reducing PCP to sym-tet. (With regard to the latter sensitivity, see Basics of Electroorganic Synthesis, D. K. Kyriacou, John Wiley & Sons, N.Y., N.Y. 1981; pp. 20, 22, 28, 54, 69, 82, 92, 102-4, 112-117, 125, 133 and 134.)
The higher current densities said by Pletcher et al to be attainable with reaction media which are emulsions are desirable but it is well known (Kyriacou, loc. cit., p. 16) that emulsions give trouble in divided cells (which would appear to be indicated for a process involving formation of zinc at the cathode and of chlorine at the anode). Efficient operation with emulsions also imposes considerably higher stirring requirements.